1. Field of the invention:
The present invention relates to a composite material, a method for controlling the thermal effects generated in a physicochemical process using said material, and applications of the material and the method.
2. Description of the Related Art
In various technical fields, the methods put into practice are based on a reversible physicochemical process that is exothermic in one direction and endothermic in the other. In this type of method, it is generally desirable to remove the heat liberated during the exothermic step, and it is necessary to supply the heat necessary for the endothermic step to obtain satisfactory results.
The storage of a gas in an adsorbent solid is widely investigated and employed. The adsorption of the gas on the solid is exothermic and the heat liberated has a detrimental effect on adsorption efficiency. Moreover, a reverse problem arises during the desorption of the gas during the regeneration step. In fact, this desorption generates an endothermic effect that is even greater when the regeneration step is carried out at high gas flow rates. This endothermic effect strongly inhibits the desorption of the gas, and the kinetics of the method are accordingly limited by the heat input necessary. The usual solutions consist, in the case of adsorption for example, in removing the heat formed to the exterior, necessitating the use of a very high thermal conductivity material as adsorbent solid. This high thermal conductivity can be obtained by adding expanded natural graphite (ENG) to activated charcoal [S. Biloé, et al., Carbon, 2001, 39(11), 1653-1662)] or by using an ENG-activated charcoal composite prepared by in situ activation (WO01/55054).
The storage of energy on composite materials containing a phase change component has also been investigated. These materials nevertheless present very poor thermal conductivities (about 0.2 W·K−1m−1). In activated charcoal-paraffin composites in which the activated charcoal is impregnated with paraffin, the confinement of the paraffin in the micropores of the activated charcoal inhibits the energy properties of phase change inherent in the paraffin [C. Chapotard, et al., (Entropie 1982; 107-108: 112-121)]. Composite materials comprising expanded natural graphite (ENG) and paraffin have also been described (X. Py, et al, International Journal of Heat and Mass Transfer, 2001, 44, 2727-2737). ENG is impregnated with the paraffin by simple capillarity. The thermal conductivity of this composite corresponds to that of ENG, which then only serves as container and thermal conductor. This composite material does not contain activated charcoal and hence does not display any adsorption capacity. Sweating of the paraffin is also observed during the use of this type of composite.
Patent WO98/04644 teaches a method and a system for storing heat or cold in a composite material comprising an expanded and compressed graphite matrix and a phase change material that can be, in particular, a congruent melting salt. The composite material is obtained by vacuum impregnation of the matrix by a salt solution or by immersion of the matrix in a salt solution. As in the previous case, the objective of this type of material is exclusively to store energy in the form of latent heat, and not to control the thermicity of a physicochemical process combining a gas with an active solid. Also observed is the presence of blooming problems of the phase change material, as in the case of paraffin.
It is well known how to prepare micronodules of various materials. For example, the encapsulation of an odorant (Migrin Oil) is described by K. Hong et al. [Materials Chemistry and Physics, 58 (1999) 128-131]. The encapsulation method consists in contacting a precondensate of melamine and formaldehyde in alkaline medium with an aqueous emulsion of Migrin Oil and 1,4-diaminoanthraquinone (DDA). Furthermore, micronodules of paraffin in a polymer envelope obtained by crosslinking a melamine resin are marketed by BASF AG. [E. Jahns, BASF, “Microencapsulated Phase Change Material”, www.ket.kht.se/Avdelningar/ts/annex10/WS_pres/Jahns.pdf]
Composite materials are used in the textile field, comprising fibers of materials conventionally used for textile fibers and micronodules of a phase change material. The micronodules can be grafted onto the fibers or distributed in the mass of material constituting the fibers. [G. Nelson, International Journal of Pharmaceutics. 2002, 242, pp. 55-62].
The problem that the invention proposes to solve is to remedy the inhibiting effect engendered by the heat liberated during the exothermic step and by the heat consumed in the endothermic step in methods involving a reversible physicochemical process that is exothermic in one direction and endothermic in the opposite direction, and which takes place in a solid material. The purpose is to control in situ the thermal effects occurring during the physicochemical process, in order to ensure isothermal operation.